1. Field of the Invention
This invention relates to novel polyol acetal compounds which are useful as nucleating agents for polyolefins. This invention also relates to polyolefin based compositions containing the polyol acetal compounds. Another aspect of this invention relates to articles of manufacture formed totally or in part from the polyolefin based composition of this invention.
2. Description of the Prior Art
The "super" or morphological structure in which the crystalline units are arranged, affects the physical properties of polyolefins. The crystalline units are arranged in polycrystalline aggregates known as spherulites. These spherulites may be detected by microscopic examination under polarized light. They are characterized by a more or less symmetrical growth in all directions from a nucleus and are composite structures made up of crystalline and amorphous regions. The number and size of the spherulites determines the texture or graininess in the bulk of the material and influences optical as well as physical properties. Physical properties improve with increasing homogeneity and fineness of the spherulitic structure throughout the bulk of the material.
To obtain optimum physical properties in articles fabricated from polyolefins, it is desirable, therefore, to produce a highly crystalline material, crystallized with an extremely fine, dense and uniform morphological structure.
Among the physical properties affected by increased crystallinity and improved morphological structure are abrasion resistance, heat distortion temperature, inherent stability or resistance to deformation, resistance to hot water, coefficient of expansion, hardness, tensile yield strength and surface hardness.
Nucleation by foreign materials has been extensively studied, especially in the case of polypropylene. For example, H. N. Beck or H. D. Led better, J. Appl. Polym. Sci. 9, 2131 (1965) and H. N. Beck, J. Appl. Polym. Sci. 11,673 (1987) checked the nucleation activity of more than two hundred substances by determining the temperature, Tcc, at which the crystallization rate on cooling is the fastest. F. L. Binsbergen, Polymer, 11, 253 (1970) extended these studies in testing two thousand substances for nucleating activity in polyethylene, polypropylene, poly(4-methyl-1-pentene) and poly(styrene). Other working nucleating agents for polyolefin are described on J. P. Mercier, Polymer Engineering and Science, 30, 270 (1990), Wijga, P. W. O. U.S. Pat. No. 3,207,735; --6; --8(1960) Wijga, P. W. O. and Binsbergen, F. L. U.S. Pat. No. 3,299,029(1961) Wales, M. U.S. Pat. No. 3,207,737;--(1961-62) Binsbergen, F. L. U.S. Pat. Nos. 3,326,880; 3,327,020;--1(1963) Kargin, V. A. et al, Dokl. Akad. Nauk. SSSR 1964, 156, 1156(transl.: Dokl. Phys. Chem, 1964, 156, 621, 644) Doring, C. and Schmidt, H. German Pat. (Federal Rep.) 1,188,279(1963) and Vonk, G. C. Kolloid Z. 1965, 206, 121.
The function of nucleating agents when cooling semi-crystalline polymers from the molten state into the solid form is to increase the number of nuclei formed in a given time interval at a predetermined temperature. The final and overall crystallinity, however, depends not only on the number of nuclei that are formed but also on the spherulitic growth rate from such nuclei. As noted above, spherulites develop with respect to a center, or nucleus, of growth. Addition of the nucleating agents thus provides a large number of sites for growth upon cooling from a melt. In order to be of practical use, such nucleating agents not only must produce a fine spherulitic structure but also must do this under conditions of rapid cooling to a temperature above the glass transition temperature of the polyolefin, i.e., they must reduce the time that is necessary under a given set of conditions for crystallization to start. This time is usually referred to as "induction time". Subsequent growth from the spherulitic center depends on the polymer chain mobility. Thus, a factor in the spherulitic growth rate is the macroscopic viscosity of the polymer and its temperature dependence. All segmental motion is "frozen in" at the glass transition temperature (Tg) and no additional crystallization occurs even when nuclei are present. This Tg is about -20.degree. C. in polypropylene.
Much of the art relating to polyol acetals relates to the use of dibenzylidene sorbitol compounds as additives in various polyolefins. U.S. Pat. No. 3.948,946 to Hofer et al; U.S. Pat. No. 4,016,118 to Hamada; U.S. Pat. No. 4,294,747 to Su; and U.S. Pat. No. 4,371,645 to Mahaffe4y, Jr. are exemplary of polyol acetal compounds known in the art.